The present invention relates to a device for controlling wheel cylinder brake pressure, and in particular to a brake pressure control device for use in so-called anti-skid control devices which decreases sliding of a car wheel by decreasing wheel cylinder brake pressure when sliding of the car wheel against road surface is going to be excessive when applying brake pressure to the wheel cylinder.
In a previous anti-skid control system, an electromagnetic opening/closing valve for high pressure connection to connect the car wheel brake to the brake master cylinder and an electromagnetic opening/closing valve for low pressure connection to connect the car wheel brake to a drain pressure or low pressure are utilized for car wheel brake pressure control (See Japanese Patent Publication No. Sho-51-6308 and Japanese Patent Publication No. Sho-62-125942).
When a brake pedal is stepped on and the brake pressure (high pressure) is applied to the car wheel brake from the brake master cylinder through the electromagnetic opening/closing valve for high pressure connection, the rotational speed of the wheel is decreased by the braking power of the car wheel. However, when either the road surface is icy or it is covered by some easily slidable material such as oil, the frictional factor of the car wheel against the road surface becomes very low and the car wheel slides on the road surface. Therefore, despite the car being advanced at a relatively high speed, the rotational speed of the car wheel is decreased rapidly by the braking power. For example, when the car wheel is stopped, the car wheel slides on the road surface, steering becomes impossible and the braking distance until the car is stopped from the brake pedal being stepped on becomes too long. In order to prevent this, when the slipping rate of the car wheel is calculated on the basis of the car wheel rotation speed and basic speed of the car and the car wheel is going to be locked on the basis of this slipping rate and acceleration of the rotational speed of the car wheel is desired in response to the requirement, the electromagnetic opening/closing valve for high pressure connection is closed, and the electromagnetic opening/closing valve for low pressure connection is opened so that the car wheel brake pressure is lowered. When the rotational speed of the car wheel is raised, the electromagnetic opening/closing valve for high pressure connection is opened, and the electromagnetic opening/closing valve for low pressure is closed, so that the car wheel brake pressure is raised (combination of decreasing pressure mode and increasing pressure mode). By repeating this operation, the car wheel brake pressure, i.e., the slip of the car wheel against the road surface, is controlled to be within a predetermined range and as a result, steering is improved and the braking distance is shortened.
In the above-described anti-skid control, since the car wheel brake pressure is shifted in binary, variation of the car wheel brake pressure is coarse. Therefore, it is also being carried out after the pressure is decreased and the acceleration of car wheel rotation has become high to a certain degree and has entered into a certain range, the electromagnetic opening/closing valves for high pressure connection and low pressure connection are both closed (cut off) and the car wheel brake pressure is held (holding) at a value of that time, and when the acceleration of the car wheel is further raised, the electromagnetic opening/closing valve for high pressure connection is opened (combination of decreasing mode, holding mode and increasing mode).
In order to control the car wheel brake pressure more smoothly, and to substitute two conventional electromagnetic opening/closing valves for one electromagnetic shifting valve, for example, in anti-skid control of Japanese Patent Publication No. Sho-62-270795 in a holding mode during transferring from the decreasing mode to the increasing mode, the pressure decreasing and the pressure increasing are shifted alternatively within a relatively short time period, and the duty cycle of the pressure increasing on said repeating (pressure increasing period of time+(pressure increasing period of time+pressure decreasing period of time).times.100%) is gradually raised. In the anti-skid control device of said Japanese patent publication Sho-62-270795, in order to make the valve device for the brake pressure control to be more simple, an electromagnetic shifting valve is utilized, in which the valve body is driven by an electric coil, so that car wheel brake is selectively connected to either the brake master cylinder or drain pressure.